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Missing bottom paint

Discussion in 'Ask All Sailors' started by Bill22113, Jan 12, 2018 at 10:34 AM. Add this thread to a FAQ

  1. Bill22113


    Joined Aug 19, 2005
    11 posts, 0 likes
    Cal 33 MkII
    US Clinton ,CT
    Last summer my Cal33 lost patches of copper bottom paint around all bronze through hulls. Had cleaned all bonding connections before launch and used aluminum prop shaft anodes instead of zinc (which never lasted a season). Aluminums less than half gone by haulout. Why the copper bottom paint loss?

  2. rgranger


    Joined Jan 19, 2010
    4,276 posts, 460 likes
    Hunter 26
    US Smith Mountain Lake
    You switched from zinc. My first guess is electrolysis. Aluminum has an electrochemical oxidation potential of -1.706 V and the copper in your paint has a reduction potential of 0.3402 V


    2(Al ---> Al+3 + 3e-) +1.706
    3(Cu+2 + 2e- --> Cu) +0.3402
    2Al + 3Cu+2 ---> 2Al+3 + Cu =2.05V

    You have a 2+ volt electrochemical difference between the copper in your paint and the aluminum sacrificial anode.

    A zinc anode has an oxidation potential of 0.762 .... almost a volt lower potential.

    If you think of voltage as being analogous to pressure..... then the higher the voltage difference, the more pressure two dissimilar metals have to react. It probably was happening with your zinc just at a slower pace. Let me ask you this.... did you have growth problems in the affected area? If not.. I would not worry about it.

  3. Bill22113


    Joined Aug 19, 2005
    11 posts, 0 likes
    Cal 33 MkII
    US Clinton ,CT
    Thanks rgranger. Perhaps I'll only use one Al anode this year. Still looking for cause of rapid zinc anode consumption in previous years but that's another topic.

  4. capta


    Joined Jun 4, 2009
    2,095 posts, 384 likes
    Pearson 530
    XX Sailing in the Windward or Leeward Islands where ever we are anchored
    Zincs can be affected by a multitude of things that have nothing to do with YOUR boat. An old engine block close to your slip, a steel boat or two on the docks, a leaking shore power connection on one of the dock pedestals. The list is endless.
    When I started sailing, there were still boats with copper sheathing on their hulls and if a steel boat came into that marina (or vice versa), everybody's zincs would go ballistic.
    It was my understanding that the alloy zincs were for fresh water only. Perhaps that thinking has changed? If you really want to know what is going on with your boat, a surveyor or marine electrician will have a meter for measuring your 'leakage', if any. Call one and get your boat tested.

  5. Gunni


    Joined Mar 16, 2010
    4,482 posts, 631 likes
    Beneteau 411 Oceanis
    US Annapolis
    Suspicion: Your newly optimized bonding system turned your boat into a more effective partner in marina stray current activity. The aluminum anodes are supposed to last longer which until you get this sorted out is a good thing for your prop and exposed drive.

  6. thinwater


    Joined Mar 26, 2011
    1,659 posts, 206 likes
    Corsair F-24 MK I
    US Deale, MD
    Google "paint burn back."

    fstbttms likes this.
  7. fstbttms


    Joined Feb 26, 2011
    1,040 posts, 73 likes
    Achilles SD-130
    US Under a boat in the San Francisco Bay Alameda, CA
    Yes, it's called "burn back" and is caused by improper priming of underwater metals.

  8. dlochner


    Joined Jan 11, 2014
    1,287 posts, 332 likes
    Sabre 362
    US Fair Haven, NY
    Capta, if the "alloy zincs" you refer to are the Aluminum and Magnesium anodes, then rest the thinking has changed. Aluminum can be used in fresh, brackish or salt water. Magnesium only in fresh water, zinc only in saltwater. has good information on this.

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